As the size of integrated circuit line widths continue shrinking to as low as forty five nanometers (45 nm), utilization of immersion lithography is becoming more and more common. To provide a better understanding of the principles of immersion lithography, a prior art dry optics lithography system will first be described.
FIG. 1 illustrates an exemplary arrangement of a prior art dry optics lithography system 100 for performing a lithography process on a photoresist material 110. As shown in FIG. 1, dry optics lithography system 100 includes a first lens 120, a second lens 130, and a third lens 140. The three lenses 120, 130, and 140 are aligned along a vertical optical axis 150 perpendicular to a top surface of the photoresist material 110. An air gap 160 is disposed between the top surface of the photoresist material 110 and the bottom surface of the first lens 120.
In an dry optics lithography system of the type illustrated in FIG. 1, the numerical aperture (NA) is given by the expression NA=sin θ, where θ is the angle between the vertical optical axis 150 and the outermost optical ray 170 that passes through the dry optics system 100. The numerical aperture (NA) is a dimensionless number that characterizes the range of angles over which an optical system can accept or emit light.
FIG. 2 illustrates an exemplary arrangement of a prior art immersion lithography optics system 200 for performing a lithography process on a photoresist material 210. As shown in FIG. 2, the immersion lithography optics system 200 includes a first lens 220, a second lens 230, and a third lens 240. The three lenses 220, 230, and 240 are aligned along a vertical optical axis 250 perpendicular to a top surface of the photoresist material 210. A gap 260 disposed between the top surface of the photoresist material 210 and the bottom surface of the first lens 220 is filled with an immersion material 270. The immersion material may be either a liquid or a solid.
According to the theory of immersion lithography, the immersion material 270 filling the gap 260 between the first lens 220 and the photoresist material 210 reduces phase error of the incident ray and helps increase depth of focus (DOF) and resolution of the optical system.
In an immersion optics system of the type illustrated in FIG. 2, The numerical aperture (NA) is given by the expression NA=n sin θ, where θ is the angle between the vertical optical axis 250 and the outermost optical ray 280 that passes through the immersion lithography optical system 200 and where the letter “n” designates the value of the index of refraction of the immersion material 270.
When the immersion material 270 is a liquid, several practical production problems may occur. These problems include a leaching effect, a liquid evaporation cooling effect, an immersion defect, and so on. For at least some of these reasons, a solid is sometimes utilized for the immersion material 270 and the process is referred to as solid immersion lithography.
In solid immersion lithography, the solid immersion material 270 is in direct contact (or in close proximity to) the photoresist material 210. This makes it difficult to have relative motion between the lens assembly and the photoresist material 210 without scratching the surface of the photoresist material 210 which makes it virtually impossible to engage in high speed scanning. This also limits the throughput of the process.
Accordingly, there is a need in the art for an improved immersion lithography apparatus and method that remedies the above described deficiencies of the prior art.